Step 1: Use Raoult's law in its direct form:
$p = x_{\text{solvent}}\,p^{\circ}$.
$x_{\text{solvent}} = 27/30 = 0.9$.
Step 2: Get the solute fraction:
Mole fractions add up to 1, so $x_{\text{solute}} = 1 - 0.9 = 0.1$.
Step 3: Why not 0.9:
The figure 0.9 is the solvent's share. The question asks for the solute, which is the smaller part, so 0.1 is the answer.
Step 4: Why the other options fail:
Option (A) 0.9 is the solvent share, which is what 27/30 gives. Option (C) 0.4 and option (D) 0.6 do not come from any ratio of 27, 30 and 3. The vapour pressure fall is only 3 mm Hg out of 30, so a solute fraction as large as 0.4 or 0.6 is not reasonable.
Final Answer:
$x_{\text{solute}} = 0.1$, option (B).
\[ \boxed{0.1 \text{ (B)}} \]